The present invention relates to a device and method for monitoring the blood pressure of a patient and, more particularly, for deriving the blood pressure from measurements performed continuously on the finger of the patient.
Noninvasive ambulatory blood pressure monitoring is currently limited to the simple measurements of systolic and diastolic blood pressures at intervals. However, it is known to clinicians that continuous waveforms of the blood pressure can provide more useful information about the patient""s cardiovascular state that are difficult to obtain from the routine antecubital pressure measurement. For example, the rate of pressure rise at the beginning of systole indicates the strength of cardiac contraction while the rate of pressure decay during end diastole can be used as a measure of peripheral vascular resistance, both of which are important parameters used in cardiovascular diagnoses. In fact, many numerical algorithms have been developed to estimate left-ventricular and circulatory parameters from the arterial pressure waveform by applying a computer model of the cardiovascular system, as described by J. W. Clark, et al., xe2x80x9cA Two-Stage Identification Scheme for the Determination of the Parameters of a Model of the Left Heart and Systemic Circulation,xe2x80x9d IEEE Trans. on Biomed. Eng., Vol. 27, pp. 20-29, January, 1980; W. Welkowitz, Q. Cui, Y. Qi and J. Kostis, xe2x80x9cNoninvasive Estimation of Cardiac Output,xe2x80x9d IEEE Trans. on Biomed. Eng., Vol. 38, pp. 1100-1105, November, 1991; M. Guarini, J. Urzua, A. Cipriano, and W. Gonzalez, xe2x80x9cEstimation of Cardiac Function From Computer Analysis of the Arterial Pressure Waveform,xe2x80x9d IEEE Trans. on Biomed. Eng., Vol. 45, pp. 1420-1428, December 1998; and E. T. Ozawa, xe2x80x9cA Numerical Model of the Cardiovascular System for Clinical Assessment of the Hemodynamic State,xe2x80x9d Ph.D. Thesis, Dept. of Health Sciences and Technology, MIT, September, 1996. Considering that heart disease is a prevalent cause of death in the modern society, it is obvious that long-term noninvasive continuous monitoring of such pressure waveforms would bring enormous improvement of the quality of healthcare at home as well as in the hospital.
A few devices have been developed for continuous monitoring of the arterial pressure waveform, yet these are either invasive or mechanically intrusive and are not designed for the long-term use. For example, Pressman and Newgard developed a noninvasive method for continuously measuring the instantaneous blood pressure by applying the coplanar measurement principle used by tonometry, as described in G. Pressman and P. Newgard, xe2x80x9cA Transducer for Continuous External Measurement of Arterial Blood Pressure,xe2x80x9d IEEE Trans. on Biomed. Eng., Vol. 10, pp. 73-81, 1961. In this method, called xe2x80x9carterial tonometry,xe2x80x9d the artery is flattened by applying external pressure non-invasively to squeeze the artery against the bone. Since the circumferential tension of the arterial wall disappears, the applied pressure to maintain the flattened shape indicates the arterial blood pressure. An array of piezoelectric transducers is used for the pressure reading. Penaz, on the other hand, proposed a new noninvasive, continuous blood pressure measuring method based on the principle of vascular wall unloading, as described in Penaz, xe2x80x9cPhoto-electric Measurement of Blood Pressure, Volume and Flow in the Finger,xe2x80x9d Digest of the 10-th Int. Conf. on Medical and Biolog. Eng., 1973. In this method, a cuff is inflated to a pressure equal to the pressure in the artery and the cuff pressure is continuously adjusted by a servo control system, which monitors the size of the artery using a photoplethysmograph. This method was further developed by Wesseling, as described in K. H. Wesseling, xe2x80x9cNon-invasive, Continuous, Calibrated Blood Pressure by the Method of Penaz,xe2x80x9d Blood Pressure Measurement and Systemic Hypertension, pp.163-175, Medical World Press, and successfully commercialized as xe2x80x9cFINAPRES.xe2x80x9d Yamakoshi and his group also developed a similar device independently by applying the vascular unloading technique, as described in C. Tase and A. Okuaki, xe2x80x9cNoninvasive Continuous Blood Pressure Measurementxe2x80x94Clinical Application of FINAPRESxe2x80x94,xe2x80x9d Japanese J. of Clinical Monitor, Vol. 1, pp. 61-68, 1990; and K. Yamakoshi, H. Shimazu and T. Togawa, xe2x80x9cIndirect Measurement of Instantaneous Arterial Blood Pressure in the Human Finger by the Vascular Unloading Technique,xe2x80x9d IEEE Trans. on Biomed. Eng., Vol. 27, pp. 150-155, 1980. The major drawback of these devices, however, is the tight confinement and mechanical intrusiveness of the sensor probes and the resultant discomfort to the patient. As stated above, these methods require a constant and continuous external pressure on the skin surface of the patient and it could cause vasospasm and pressure drops in the peripheral artery, as described in A. Kawarada, H. Shimazu, H. Ito, and K. Yamakoshi, xe2x80x9cAmbulatory Monitoring of Indirect Beat-to-Beat Arterial Pressure in Human Fingers by a Volume-Compensation Method,xe2x80x9d Med Biol Eng Comput, Vol. 34, pp. 55-62, January 1991. For long-term, ambulatory blood pressure monitoring, a new method for the noninvasive and non-intrusive continuous measurement is preferable.
In accordance with one aspect of the invention, a system for monitoring a blood pressure of a patient uses a first photoplethysmograph proximate to a finger of the patient for providing a measure of change in the arterial diameter at a first location of a specified artery of the patient. A second photoplethysmograph proximate to the finger of the patient and displaced relative to the first photoplethysmograph provides a measure of change in the arterial diameter at a second location of the specified artery of the patient. An electrical impedance plethysmograph in electrical contact with the finger of the patient provides a measure of change in the electrical impedance of an arterial segment between the first and the second locations of the specified artery. A controller derives a measure of the blood pressure of the patient based on the measures of change in the arterial diameter at the first and second locations of the specified artery and the measure of change in the electrical impedance of an arterial segment.
In a further related embodiment, the first photoplethysmograph is borne by the patient on a finger ring. In another related embodiment, the first photoplethysmograph is borne by the patient on a first band of a finger ring and the second photoplethysmograph is borne by the patient on a second band of the finger ring. In some embodiments, a transmitter may optionally be used for transmitting the measure of the blood pressure of the patient to a remote location.
In another embodiment, a system for monitoring a blood pressure of a patient uses a monitor having a first and a second band to be worn by the patient on a single finger. The monitor has a first photoplethysmograph disposed on the first band for providing a first signal based on a first arterial diameter of the patient, a second photoplethysmograph disposed on the second band for providing a second signal based on a second arterial diameter of the patient, and an electrical impedance plethysmograph disposed on the first and second bands for providing a third signal based on the electrical impedance of the a segment of an artery of the patient. A controller analyzes the first, second, and third signals and determines a measure of the blood pressure of the patient.
In accordance with another embodiment, a method for monitoring the blood pressure of a patient derives a measure of change in both the diameter of the first and second end of a segment of an artery of the patient. A model of arterial blood flow is applied to the derived measures of change in the diameters of the first and second ends of the arterial segment and the volume of the segment for calculating the instantaneous blood pressure of the patient. In a related embodiment, the step of deriving the measure of change in the diameter of the first end of a segment of an artery includes receiving a signal of a first photoplethysmograph. In another related embodiment, the step of deriving the measure of change in the diameter of the second end of a segment of an artery includes receiving a signal of a second photoplethysmograph. In some embodiments, a Kalman filter is used for estimating internal state variables.